MOS devices are more and more employed in power electronics thanks to the simplicity of the circuits necessary for their driving operation and to their switching performances.
Among power MOS devices, those which can be directly driven by logic-level signals (called logic-level power MOS) hold a significant market share. In their most common use, logic-level power MOS devices can be directly driven by a microprocessor.
Since it is known that most logic families have logic-level voltage values compatible with those of the TTL technology, it follows that logic-level power MOS devices must have threshold voltage values ranging from 1 V to 2 V: in fact, they must remain in the "off" state for gate driving voltages between 0 V and 0.8 V (corresponding to the "0" logic level), while they drain the nominal current when the gate voltage is higher than 2.4 V (corresponding instead to the "1" logic level).
Due to the low value of the threshold voltage, these devices are labeled with the letters LT (Low Threshold).
In order to achieve said threshold voltage values, it is necessary for logic-level MOS power devices to have a gate oxide layer that is rather thin, with thickness typically ranging from 30 nm to 80 nm. Such a thin layer can be permanently damaged if a voltage higher than a few tens of volts is applied to the gate of the device. For this reason logic-level power MOS are particularly affected by Electro-Static Discharges ("ESD") problems.
To prevent ESD failures, manufacturers of logic-level power MOS devices provide said devices with integrated structure protections, usually made up of diodes.
Such protections, however, set two opposing kind of limits: the first is represented by the breakdown voltage of each of said diodes, while the second deals with the series resistance introduced by the diodes.
Each diode should have a breakdown voltage higher than the maximum driving voltage of the power MOS device, in order to prevent it from draining current from the driving circuit of the power MOS device. Since diodes obtained from p.sup.+ /n.sup.+ junctions exhibit low breakdown voltages (with values even below 5 V), it is necessary to connect a plurality of such diodes in series.
Another possibility is represented by the use of p/n.sup.+ junction diodes, which generally have breakdown voltages of the order of 10 V.
If, however, the series resistance introduced by the protection structure is not negligible, it can give rise to a voltage drop able to damage the protected power device during an electro static discharge.